Process for improving the properties of a compacted form of sodium cyanide crystals



PROCESS FOR IMPROVING THE PROPERTIES OF A COMPAGTED FORM OF SODIUMCYANIDE CRYSTALS Filed Jan. 25, 1962 Sept. 14, 1965 c P GREEN 54 I "I Jm 70 r 9 60 58 i E IIJY .II E

INVENTOR CHARLES R GREEN BY fiwim ATTORNEY United States Patent3,206,278 PROCESS FOR IMPROVING THE PROPERTIES OF A COMPACTED FORM OFSODIUM CYANIDE CRYSTALS Charles P. Green, Memphis, Tenn., assignor to E.I. du Pont de Nemours and Company, Wilmington, Del., a corporation ofDelaware Filed Jan. 25, 1962, Ser. No. 173,853 2 Claims. (Cl. 23-79)This application is a continuation-in-part of application Serial No.716,501, filed February 20, 1958, now abandoned.

This invention relates to compacted forms of sodium cyanide crystals,and is more particularly concerned with a heat-treating process forimproving briquettes which have been formed by pressing together finelydivided sodium cyanide crystals, and with the improved briquettesproduced by said process.

A large amount of sodium cyanide is supplied for industrial use inmolded forms. Such molded forms are greatly preferred for general usebecause of the ease and safety in handling in comparison with finelydivided forms which are dusty and relatively hazardous. Sodium cyanideproduced by the dry Castner process is obtained directly in the moltenstate and is, therefore, cast into a molded form. customarily, this formis of spherical shape weighing about one ounce and referred to as eggs.Sodium cyanide produced by the newer wet process is recovered from anaqueous solution by crystallization and drying and obtained in the formof finely divided crystals. It has not been found practical to melt andcast these crystals into molded forms such as eggs due to the extra costinvolved and because product darkening and product decomposition occur.Instead, the substantially dry crystals are pressed into a compactedform, usually as the finely divided crystals come from a dryer at atemperature of about 140-160 C. Obviously, the crystals might be pressedinto forms of many diiferent shapes and sizes but customarily is pressedinto a pillow shape Weighing approximately one ounce and referred to asbriquettes. Although the present invention concerning the improvement ofcompacted forms of sodium cyanide crystals is applicable to compactedforms of sodium cyanide crystals of any shape or size, as a matter ofconvenience it will be described hereinafter in connection withbriquettes.

Although briquettes formed as above described can currently be moreeconomically produced than cast eggs, unfortunately, they frequentlygive unsatisfactory results in processes for which cast eggs have beenquite satisfactory, It has now been found that the difficulty usuallyencountered in using briquettes instead of cast eggs results fromabsorption of moisture upon exposure to the atmosphere. This isparticularly the case in the metal treating industry where sodiumcyanide additions are made to fused carburizing and nitriding baths.When the moisture content of the sodium cyanide is as high as 0.5% byweight, excessive flame and sputtering occur and a layer of foam formson the surface of the molten bath, frequently overflowing the pot andspilling into the heating elements of the furnace. This is, of course,undesirable and hazardous. Even though the sodium cyanide briquettes aredry when shipped to the user, most shops engaged in this operation usefrom the same container over a long enough time for an objectionableamount of moisture to be absorbed from the shop atmosphere. Also,absorbed moisture is often objectionable when using sodium cyanide as araw material in chemical syntheses because the yields are adverselyaffected by the presence of moisture. Moreover, since the briquettes areformed by merely pressing together fine crystals of sodium cyanide, saidbriquettes 3,206,278 Patented Sept. 14, 1965 have limited mechanicalstrength and are considerably more fragile in use than cast eggs inwhich the sodium cyanide is fused.

It is an object of this invention, therefore, to provide a process, forimproving the properties of sodium cyanide briquettes. Another object isto provide briquettes which have low moisture absorptioncharacteristics. Another object is to provide briquettes suitable foruse in fused metal-treating baths. Another object is to providebriquettes of improved mechanical strength and appearance. A stillfurther object is to provide briquetes in which the amount of sodiumformate present as an impurity is minimized.

In accordance with the present invention, it has been found that theproperties of sodium cyanide briquettes are greatly improved by heatingfor at least 15 minutes at temperatures in the range of 3S0450 C., withsufiicient free alkali metal hydroxide present in the sodium cyanide toprovide 0.1-l.0% by weight in the product, the briquettes being cooledthereafter Without further treatment. The treated briquettes arecharacterized in having a low moisture absorption tendency and are quitesuitable for use in fused metal-treating baths. The treatment makes thebriquettes significantly harder and more resistant to breakage orformation of powder. The appearance of the briquette is also improved,especially by being markedly whitened to a uniform pure white.

The process will be described with reference to sodium cyanidebriquettes, but is also useful for improving the properties of othercompression molded alkali metal cyanides, such as potassium cyanide. Theimprovement in properties will be indicated principally by reference tothe elfect on moisture absorption and the effect on suitability for usein fused baths for metal heat-treating purposes, since numericalcomparisons of the effect of the treatment are readily made. However,significant improvements in the other properties mentioned above arealso obtained. Moreover, it has been found that the process of theinvention has a significant effect in reducing the amount of sodiumformate present so that the treated briquettes are characterized inexhibiting a much lower content of sodium formate than the untreatedbriquettes. Sodium formate is invariably present in briquetes formedfrom sodium cyanide crystals produced by the Wet process since thismaterial is unavoidably formed as a result of the reaction between waterand sodium cyanide, and is usually present in amounts in excess of 0.3%by weight in the initially formed briquettes. Besides lowering thepurity of the product, sodium formate is found to be a veryobjectionable impurity since it has been found to contribute in itselfto flaming and foaming when briquetes are added to heat-treating bathsand, in addition, is found to adversely affect yields in variouschemical syntheses when sodium cyanide is used as a reactant material.For briquettes used in heat-treating baths, it has now been found highlypreferable to maintain the content of sodium formate below 0.2% byweight and contents of sodium formate below this level may be readilyobtained in briquettes treated by the process of the present invention.

Heating the briquettes in the temperature range of 350- 450 C. is quitecritical in order to lower the moisture absorption characteristics andto effect the other stated improvements in properties. The briquettesmay be heated in any convenient way for at least 15 minutes in thistemperature range as in the presence of air at substantially atmosphericpressure, and are then cooled or allowed to cool. Since moisture reactswith sodium cyanide to form sodium formate and ammonia at anincreasingly rapid rate as the temperature is raised, it is preferablethat the briquettes be substantially dry and be surrounded by asubstantially dry atmosphere during the cyanide polymers, etc.

heating and cooling steps. Briquettes which contain appreciable moistureshould first be dried at temperatures below about 200 C. The briquettescan be heated in a closed container or in a stream of hot air or otherinert gas. The process is most conveniently carried out with acontinuous rotary oven which is indirectly heated or supplied with hotair to heat the briquettes to 350450 C. as the briquettes are fedthrough the heating machine. Atmospheric air heated to a temperatureabove 350 C., preferably 400450 C., is suitable for this purpose withoutpreliminary drying.

A- minimum of 0.1% by weight sodium hydroxide or other alkali metalhydroxide must be present in the briquettes throughout the treatment toprevent darkening. For briquettes that are to be used to form aqueoussolutions 0.3% by weight of alkali metal hydroxide is preferred in orderto avoid hydrolysis and formation of On the other hand, the purity ofthe product should not be lowered by the presence of more than about1.0% by weight of sodium hydroxide, and it is generally preferable tohave about 03-06% by weight present in the final product. It has beenfound in accordance with the present invention that a larger amount ofhydroxide must be provided at the start of the process because of thesodium formate which is present in the untreated briquette, due to thefollowing reaction which apparently occurs during the heating within the350450 C. temperature range:

In addition to the sodium formate initially present as an impurity,additional formate may be formed by any moisture present which reactswith the sodium cyanide. The net effect of any moisture present in theuntreated briquette, therefore, is as follows:

The percent by weight of sodium hydroxide which will be used up by thesetwo reactions is approximately (40/58) (percent by weight sodium formatepresent) plus (40/36) (percent by weight moisture present). However, theamount of sodium hydroxide actually used up need not be estimated inthis way, since it can easily be determined by trial under the specificconditions to be employed.

The reason for the improvement achieved by the process of this inventionis not completely understood but is believed to result from crystalgrowth which in turn reduces the degree of porosity of the briquette.While the untreated briquette is of stable form and seems solid, pressedbriquettes can be shown to be relatively porous by tests with wet dyes,by water absorption rates or by treatment with carbon dioxide. At 350450C. temperature of the invention which is well below the melting pointfor sodium cyanide, namely, 560 C., the fine crystals apparently growtogether to reduce the porosity and, surprisingly, while remaining freeof fused sodium cyanide, the treated briquettes gain significantmechanical strength approaching the strength of cast eggs containingfused sodium cyanide throughout the molded form. As a result of thetreatment, the moisture absorption of briquettes is readily reduced toless than one-half that of untreated briquettes upon exposure to commonatmospheric conditions. As shown in the subsequent examples, themoisture absorption of treated briquettes is less than 0.3% by weightand, depending upon the conditions, may even be less than 0.1% byweight. The absorption inside the briquettes is most sharply reduced.Moisture absorbed on the outside of briquettes is driven off withoutdifficulty when briquettes are added to a fused heat-treating bath,whereas moisture absorbed within the briquettes has been found to causeobjectionable foaming.

Specific embodiments of the invention will now be described withreference to the figures of the drawing wherein FIGURE '1 is a sectionalrepresentation of a rotary apparatus suitable for continuous processingof briquettes in accordance with the invention, the section being takenalong the axis, and FIGURE 2 illustrates an apparatus, likewise insection, which is suitable for batch operations.

The apparatus illustrated in FIGURE 1 is an indirectly heated, rotaryshell heating machine of conventional type suitable for continuousoperation. A horizontal rotating shell 10 of generally cylindrical shapecontains the briquettes 12 while they are heated. The briquettes are fedinto the shell at one end through the opening in end cover 14. Incontinuous operation, they pass along the length of the shell and areremoved through the opening in the other end cover 16 after sufficienttreatment, passing through chute 18 to customary handling equipment.Cooling by natural heat transfer is usually preferable, although meansfor more rapid cooling can be provided. The shell is rotated by motor 20acting through reducing mechanism 22 and gear 24 mounted on hollow axle26 attached to the shell. Duct 28 is provided to feed air into the shellthrough the hollow axle, if a circulation of air is desired, e.g., fordirect heating of the briquettes with hot air. When air is supplied inthis way, it passes through the shell and out through the opening incover 14 at the opposite end.

The shell is surrounded by a housing 30 which supports the shell onbearings 32, indicated at each end of the shell. A gas burner 34 isprovided for generating hot flue gases within the housing. The hot fluegases pass around the autside of the shell and leave the housing throughduct 36. Obviously, other means for indirect heating of the briquettescan be used instead of the burner 34, or indirect heating can be omittedif the briquettes are heated by direct contact with hot air introducedthrough duct 28. The temperature of the briquettes is measured by athermocouple 38 connected to a recorder 40. The temperature of the hotgases within the housing is measured by thermocouple 42 connected torecorder 44. Flights or paddles 46, mounted on the inside of the shell,improve the tumbling action in order to heat the briquettesmore'eflectively.

All percentages stated in the following examples are percents by weightunless otherwise indicated.

EXAMPLE 1 Briquettes, each weighing approximately 1 ounce, were heatedin an apparatus of the type disclosed above. A 28-inch diameterstainless steel shell, 46 inches long, was used. Two flights wereprovided to give a mild tumbling action. The shell was rotated at 3revolutions per minute. A gas burner was used to raise the temperatureof the flue gas leaving the housing to about 400 C. The briquettes hadpreviously been dried at 180 C. to 0.01% moisture and analyzed 97.3%NaCN, 0.97% NaOH, 0.97% Na CO and 0.28% HCOONa. After heating forminutes the temperature of the briquettes was 300 C. and a portion wasremoved. The heating was continued for an additional 40 minutes, to heattheremaining briquettes at about 350 C. for 15 minutes, and they werethen removed. The briquettes were cooled by natural heat transfer andanalyzed for moisture content. The briquettes were then exposed to anatmosphere of 62% relative humidity at 77 F. for 4.5 hours, and thenanalyzed for moisture content again and tested by adding 10 pounds tothe pot of an industrial heat-treating bath at 1480-1500 F. The resultsof the analyses and tests are shown in Table I.

EXAMPLE 2 Example 1 was repeated but with the gas burner adjusted toraise the temperature of the flue gas to about 450 C. After heating for90 minutes the temperature of the briquettes was about 350 C. Theheating was continued for 15 minutes, during which time the briquetteswere at 350-370 C., and a portion was removed.

The heating was then continued for an additional 45 minutes to raise thetemperature of the briquettes to 400 C. The briquettes were then cooled,analyzed for moisture, exposed to an atmosphere of 62% relative humidityfor 4.5 hours, analyzed again for moisture and tested as in Example 1.The results are shown in Table I.

The treated briquettes were pure white in each instance. The moisturecontent of the treated briquettes before exposure to the moistatmosphere was below 0.05% by weight for each run reported in Table I.

The above results show that heating to 350400 C. for at least minutesremarkably reduced the moisture absorption, and the amount of foamproduced in the heat-treating bath, in comparison with merely heating to300 C. The performance of briquettes heated at 350 C. for 15 minutes wasequal to that of cast eggs in heattreating operations, and that of thesamples heated to 370-400" C. was even more satisfactory. The comparisonsample, heated only to 300 C., absorbed nearly 3 times as much moisturein the short exposure to atmosphere at typical humidity-temperatureconditions and was quite unsatisfactory for heat-treatment purposes.This clearly indicates the criticality of operating the process of theinvention at a temperature of at least 350 C. in order to improve theproperties of briquettes.

From tests made similar to Example 1 and 2, it was found thattemperatures above 450 C. are undesirable since discoloration of thebriquettes occur above this temperature. Furthermore, it is an importantadvantage to remain well below the melting point of sodium cyanide toavoid the softening or the melting of the briquettes to a point wherethey stick or adhere to one another.

Briquettes can also be processed satisfactorily in the stationary directheating oven shown in FIGURE 2. A cylindrical metal container 50 isprovided with a tight fitting lid 52, both being covered with insulation54. The briquettes 56 are supported on a false bottom formed by 8 meshscreen 58 resting on angle iron supports 60. Hot air is supplied to thecontainer through duct 62 in the lid 52. A thermometer 64 is provided inthis duct to measure the entering air tempearture. The air passesdownward through the briquettes into the space below the false bottomand is withdrawn through duct 66 in the bottom side of the container.The temperature of the briquettes is measured with a thermocouple 68connected to a recorder 70.

EXAMPLE 3 Briquettes of the l-ounce size were heated for 15 minutes at450 C. in the oven disclosed above with reference to FIGURE 2. Thestarting briquettes, which had previously been dried at 180 C., analyzed97.1% NaCN, 0.55% NaOH, 1.24% Na CO 0.37% HCOONa, 0.12% NaCNO and 0.7p.p.m. Na S. After heating at 450 C. the briquettes were pure white andanalyzed 96.7% NaCN, 0.10% NaOH, 2.41% Na CO 0.09% I-ICOONa, 0.09%NaCNO, and 0.6 p.p.m. Na S. When exposed to an atmosphere of 65%relative humidity at 77 F. for 2.5 hours, the treated briquettesabsorbed only 0.07%

moisture. By comparison, the initial briquettes absorbed 0.48% moisturein this test. Both the treated and the initial briquettes had a moisturecontent below 0.05% before the exposure test. The treated briquettesafter exposure to the moisture test were entirely satisfactory whentested for heat treatment use. There was no foam when 9 pounds wereadded to an industral heat-treating bath at a temperature of 1520 F. Onthe other hand, the untreated briquettes were completely unsatisfactoryfor this purpose after the short exposure to 65 relative humidity.Addition of only 5 pounds to the bath produced 4 inches of foam andcaused the pot containing the treating bath to overflow.

It is preferable to provide for uniform contact of the briquettes withhot air, by tumbling the briquettes as in Examples 1 and 2, or by forcedflow of hot air through the briquettes as in Example 3, or in both ways,since the briquettes will then be heated more effectively to therequired temperature range. However, the briquettes can also be heatedin a stationary closed container.

EXAMPLE 4 Briquettes were processed as in Example 3 except that thebriquettes were in a closed container placed Within the oven and wereheated for various times at 400 C. The corresponding analyses were asfollows:

Time 111 Percent Minutes at Percent Percent Percent Percent H 0 400 C.NaON NaOH N21100:; IICOONa After Exposure The briquettes were pure whiteafter heating.

Since many different embodiments of the invention may be made withoutdeparting from the spirit and scope thereof, it is to be understood thatthe invention is not limited by the specific illustrations except to theextent defined in the following claims.

I claim:

I. A process for improving the properties of a compacted form of sodiumcyanide crystals of at least 96.0% by weight purity and containing atleast 0.1% by weight of alkali metal hydroxide which comprises heatingsaid compacted form for at least 15 minutes at a temperature below thefusion point within the range of from 350-450 C., and then cooling saidcompacted form at the conclusion of said treatment; the amount of saidalkali metal hydroxide being sufficient to provide 0.11.0% by weight insaid compacted :form after said treatment.

2. A process for improving the properties of a compacted form of sodiumcyanide crystals of at least 96.0% by weight purity and containing atleast 0.3% by weight of alkali metal hydroxide, which comprises heatingsaid compacted form for at least 15 minutes at a temperature below thefusion point within the range of from 350- 450 C., and then cooling saidcompacted form at the conclusion of said treatment; the amount of saidalkali metal hydroxide being sufficient to provide 03-06% by weight insaid compacted form after said treatment.

References Cited by the Examiner UNITED STATES PATENTS 859,482 7/07 Bueb2384 X 912,538 2/09 Bueb 2379 X 1,262,057 4/ 18 Kaufman 2379 2,365,41712/44 Kusman 2379 2,710,788 6/55 Copelin 2384- 2,773,752 12/56 Kremer etal. 2384 X MAURICE A. BRINDISI, Primary Examiner.

1. A PROCESS FOR IMPROVING THE PROPERTIES OF A COMPACTED FORM OF SODIUMFORM OF SODIUM CYCRYSTALS OF AT LEAST 96.0% BY WEIGHT PURITY ANDCONTAINING AT LEAST 0.1% BY WEIGHT OF ALKALI METAL HYDROXIDE WHICHCOMPRISES HEATING SAID COMPACTED FORM FOR AT LEAST 15 MINUTES AT ATEMPERATURE BELOW THE FUSION POINT WITHIN THE RANGE OF FROM 350-450* C.,AND THEN COOLING SAID COMPOUND FORM AT THE CONCLUSION OF SAID TREATMENT;THE AMOUNT OF SAID ALKALI METAL HYDROXIDE BEING SUFFICIENT TO PROVIDE0.1-1.0% BY WEIGHT IN SAID COMPACTED FORM AFTER SAID TREATMENT.